(1) Field of the Invention
This invention relates to a liquid jet recording head and a substrate to be used for said recording head, and particularly to a liquid jet recording head which boils a liquid for recording by permitting heat energy to act on the liquid, thereby jetting (discharging) droplets to effect recording, and a substrate including an electricity-heat converter which generates the above heat energy corresponding to passage of current.
(2) Related Background Art
The system of discharging a liquid by utilizing abrupt growth (expansion) and shrinkage of bubbles generated by permitting heat energy to act on the liquid has been known in the art (U.S. Pat. Nos. 4,723,129, 4,740,796, etc.).
This system is suitable for high speed recording, which is a system extremely suited for higher densification, higher image quality, and is attracting attention particularly in recent years.
Desired properties for the liquid jet recording head or the electricity-heat converter to be used in this system, include a high response characteristic during high speed driving, capability of sufficient heating for boiling of a liquid, and in addition thereto, high durability. For that purpose, various improvements have been done in aspects of material and constitution.
For example, Japanese Patent Publication No. 59-34506 discloses, in order to enhance response characteristic and heating performance, an electricity-heat converter which has a lower layer, a heat-generating resistor layer and an upper layer, and also disclose the conditions which should be satisfied in relations to the thickness and the material constants of the respective layers.
Japanese Laid-open Patent Application No. 60-236758 discloses a protective layer which is made thinner on the heat-generating portion for enhancing durability.
During repeated generation and disappearance of bubbles concerned with liquid discharging (main bubbles or primary bubbles), if there is a portion of a substrate higher in temperature than the heating limit temperature, and that position is different from the portion where the mainbubbles defoam on the heat-acting portion, there will occur a phenomenon that secondary bubbles in streaks remain along the flow directional at that position. Since cavitation of such secondary bubbles is very great as compared with that of main bubbles, it may sometimes deteriorate the upper protective layer at that portion, even deteriorating the electricity-heat converter to reduce durability.
In the invention disclosed in Japanese Laid-open Patent Application No. 62-103148, by calling attention to the fact that the central part of the heat-acting portion becomes high in temperature when the upper layer and the lower layer of the electricity-heat converter are uniform in thickness, the central region of the heat-acting portion of at least one of the lower layer and the upper layer of the electricity-heat converter is made thinner in film thickness than other regions. Thus, heat dissipatability at that portion is enhanced, and during driving (during current passage through the electricity-heat converter), uniform temperature elevation is effected over the central part and the peripheral part of the heat-acting portion. Further, during defoaming of main bubbles after driving, the temperature of the central part of the heat-acting portion drops to the heating limit temperature or lower.
Also, in Japanese Laid-open Patent Application No. 59-95155, in order to prevent the above cavitation damage, an electroconductive region is provided at the central part of the electricity-heat converter (resistor), and that part is adapted to be not concerned with foaming, namely so that an annular bubble may be formed at the portion surrounding that portion, and a plurality of small bubbles may be distributed randomly on the heat-acting portion during defoaming.
However, in a recording head having an electricity-heat converter as the discharging energy generating means, in addition to the above conditions, high reproducibility of boiling is demanded.
According to the present inventors of the present application, it has been confirmed that, when a liquid is boiled repeatedly, and bubbles generated by a previous driving signal (heating pulse) of to the electricity-heat converter disappear, microscopic residual gas attaches randomly on the surface of the electricity-heat converter, which becomes the foaming nucleus at the initial bubble generation stage in the subsequent pulse heating, whereby reproducibility may not be ensured. However, this point has not be particularly considered in the prior art.
If the boiling phenomenon is not stabilized, the bubbles generated will not be constant in shape and size, and therefore variance occurs in droplet diameter and discharging speed, which can further bring about such problems as lowering in image quality.